How to do a round railing/ fence
Please try following steps

Please select the set of edges on the top of your tank, using Alt + click on the edge as show in image below:

Hit Shift + D to duplicate edges
Hit p to seperate the edges
Go to the object mode and you will get a duplicate in outliner
Go to edit mode - Select edges using Alt + click
Extrude a twice as shown in the picture below:

Select alternative faces and hit X and delete Only Faces

You can do it as a single obect too, I hope it help you